1. Field of the Invention
The invention relates to cordierite forming batch mixtures that are capable of being sintered at greatly reduced sintering temperatures to form cordierite ceramics, and to cordierite bodies, having high thermal shock resistance, formed from the batch mixtures at reduced sintering temperatures.
2. Discussion of the Related Art
The exhaust gases emitted by internal combustion systems utilizing hydrocarbon fuels, such as hydrocarbon gases, gasoline or diesel fuel, can cause serious pollution of the atmosphere. Among the many pollutants in these exhaust gases are hydrocarbons and oxygen-containing compounds, the latter including nitrogen oxides (NOx) and carbon monoxide (CO). The automotive industry has for many years attempted to reduce the quantities of gaseous emissions from automobile engine systems, the first automobiles equipped with catalytic converters having been introduced in the mid 1970""s.
Cordierite substrates, typically in the form of a honeycomb body, have long been preferred for use as substrates to support catalytically active components for catalytic converters on automobiles, in part, due to the high thermal shock resistance of cordierite ceramics. The cordierite substrates are coated with noble metal catalysts and placed in the path of the exhaust effluent of an automobile engine where the catalysts may act to convert hydrocarbons, CO and NOx to the non-toxic by-products water, carbon dioxide and reduced nitrogen species.
It is well known that the production of sintered cordierite (2MgO.2Al2O3.5SiO2) ceramics typically involves the use of mineral batches containing sources of magnesium, aluminum and silicon, such as clay and talc. U.S. Pat. No. 3,885,977 discloses the manufacture of thermal-shock-resistant cordierite honeycomb ceramics from clay/talc batches by extruding the batches and firing the extrudate to provide ceramics with very low expansion coefficients along at least one axis. Disclosed therein, is the principle, which has been used commercially since, that in order to yield substantially complete reaction to the cordierite phase the firing of the cordierite body should occur between 1340xc2x0 C. to 1450xc2x0 C. for times between about 6-12 hours. In this commercially used sintering process, growth of the cordierite crystals typically begins at about 1250xc2x0 C.
More recently it has been disclosed that lowered firing temperatures for the formation of cordierite can be obtained through a variety of different methods.
U.S. Pat. No. 4,888,314 discloses the preparation of a mixed alcoholic solution of magnesium, aluminum and silicon salts, all soluble in alcohol or a solvent miscible with alcohol. A hydrolyzing agent is thereafter added to the solution to form a gel, and the gel is thereafter subjected to a first thermal treatment at a temperature not exceeding 450xc2x0 C. and to a second thermal treatment between 450xc2x0 C. and 900xc2x0 C. resulting in the formation of a cordierite type ceramic powder. The cordierite type ceramic powder is sintered at a temperature between about 800xc2x0 C. to 1050xc2x0 C. resulting in a cordierite ceramic body having a coefficient of thermal expansion of less than about 10xc3x9710-xe2x88x927/xc2x0 C.
U.S. Pat. No. 5,173,455 discloses the formation of Group IIA-Group IIIA metal-silicon mixed liquid alkoxide which is hydrolyzed, distilled, dried, milled and thereafter doped with boron and phosphorous and sintered at a temperature of between about 825xc2x0 C. to 875xc2x0 C. to form crystalline cordierite ceramics.
Lastly R. W. Dupon et al. teach in xe2x80x9cLow Temperature Route to Cordierite Ceramics Using a Reactive Liquid Phase Sintering Aid, Dense Body Preparation and Green Tape Fabrication (J. Am. Ceram. Soc., 73 [2] 335-339, 1990) the use of 2-10 atomic percent Bi, equivalent to 8.05 to 30.47 weight percent Bi2O3, as a flux to promote the formation cordierite at temperatures of about 1000xc2x0 C. from a mixture of colloidal silica and colloidal spinel. The dense cordierite bodies disclosed therein exhibit coefficients of thermal expansion of 15xc3x9710xe2x88x927/xc2x0 C. over the temperature range of 25 to 400xc2x0 C.
While the foregoing references indicate that there have been some attempts and some progress in reducing the sintering temperatures of cordierite ceramics, it is evident that still further work is required to find simpler, and lower sintering temperature methods of forming cordierite ceramics having low coefficients of thermal expansion and the requisite adequate thermal shock resistance necessary for high temperature applications required today. Ideally, it would be useful to have sinterable ceramic batch compositions simply capable of being fired to form cordierite bodies having a low coefficient of thermal expansion, at temperatures of no greater than about 1300xc2x0 C. To date, no combination of sintering aids or methods have been discovered which are effective to reliably produce low CTE cordierite ceramics at these low sintering temperatures.
It has now surprisingly been found that the when specific metal oxides are utilized in the preparation of cordierite bodies, the above mentioned low firing temperatures can be utilized to form low thermal expansion cordierite bodies. Specifically, it has been found that the addition of a metal oxide to a cordierite-forming raw material batch mixture can be utilized to produce cordierite bodies at sintering temperatures of no greater than about 1300xc2x0 C.
The sintered ceramic article of the invention exhibits a primary crystalline phase of cordierite and an analytical oxide composition, in weight percent, of 44-53% SiO2, 30-38% Al2O3, 11-16% MgO and 0.05 to 10% of a metal oxide. The ceramic article exhibits a coefficient of thermal expansion in at least one direction no greater than about 15.0xc3x9710xe2x88x927/xc2x0 C. over the temperature range of about 25xc2x0 C. to about 800xc2x0 C. The sum of the weight percentages of residual mullite, corundum, and spinel, as measured by X-ray diffractometry of the crushed and powdered body, is not greater than 15%.
This invention additionally relates to batch mixture for use in preparing a cordierite ceramic body having as its primary crystalline phase cordierite and comprising an analytical oxide composition, in weight percent, of 44-53% SiO2, 30-38% Al2O3, 11-16% MgO, the batch mixture comprising the following:
(a) a raw material batch mixture for forming the cordierite ceramic body, comprising two or more compounds which serve as an alumina source, a silica source and a magnesia source, and at least one metal oxide source in an amount to result in the cordierite body comprising, on an analytical oxide basis, in weight percent, between about 0.05-10% of the metal oxide;
(b) an organic binder component in an amount between 2 and 50 parts by weight based on the total raw material mixture.
This invention also relates to a method of producing a ceramic body having cordierite as its primary phase, comprising selecting the aforementioned raw materials, including the metal oxide source and adding an organic binder system to the raw material mixture and thereafter kneading and shaping the mixture to form a green body. The method involves thereafter drying and firing the green body at a time and at a temperature of no greater than about 1300xc2x0 C. to result in a sintered ceramic body in which the sum of the residual mullite+spinel+corundum is no more than 15% and that exhibits the aforementioned coefficient of thermal expansion.
Metal oxide sources suitable for use in the above inventions include the oxides or oxide-forming compounds of the metals selected from the group consisting of molybdenum, tungsten, bismuth, copper, yttrium, lanthanide metals and boron.